The present invention relates to wireless communications in general and, in particular, to a satellite communications network.
A typical capability desired from broadband satellite services is the reliable recovery of payload data transmitted via a satellite through burst transmissions. Among other attributes, the burst transmission may be characterized by a transmission frequency (e.g., a carrier frequency) and a transmission time. The burst may be composed of unique word (UW) symbols (e.g., a preamble, post-amble, spread-amble, etc.), signaling header symbols, and payload data symbols. Reliable recovery of the payload data may involve reliable frequency detection and transmission start time detection through the UW detection.
One technique includes detecting UWs in burst transmissions using coherent detection, in which the UW symbols are detected only after frequency detection is complete. For example, when signal-to-noise ratio (SNR) is sufficiently high, burst length is sufficiently long, etc., the frequency and/or phase offset of the burst transmission may be determined based on payload data symbols, and then used to help detect the UW symbols. Typically, because the detection is coherent, the UW symbols may be relatively short and the burst transmission efficiency may be relatively high. However, in environments exhibiting low signal-to-noise ratios or when the burst is short, starting with frequency detection may generate unreliable results. For example, Fourier transforms may generate “fake” peaks (e.g., where there are high noise frequency components), and as-yet-undetected UW symbols cannot be exploited to assist frequency detection. Subsequently, if the frequency detected is incorrect, in turn, the beginning of the transmission cannot be correctly identified.
Another technique for detecting transmission frequencies and UWs in burst transmissions is referred to as differential detection, in which the UW symbols are detected prior to obtaining a reliable frequency reference. Often, differential detection is used when frequency detection is difficult, for example, when signal-to-noise ratios are low for the burst transmission. Typically, the reliability of differential detection techniques depends on using relatively long UW symbols, which may reduce the bandwidth efficiency of the burst transmission.
Reliably recovering payload data from burst transmissions, then, may typically entail a trade-off between efficiency and reliability. Higher reliability in noisy environments may involve differential detection using long UW symbols. Therefore, burst transmission data recovery techniques that are both reliable and efficient may be beneficial in a range of satellite communications systems.